Generally, a synthetic resin lens is advantageous in that it has a low density and is highly resistant to mechanical shock but it is, on the contrary, deficient in that it has poor wear-resisting properties and has low surface hardness.
It has been proposed hitherto, to overcome these deficiencies, to apply on the surface of the resin lens a glass coating film, and it has been conventional in this case to form the coating by the usual vacuum vapor deposition treatment. The product thus obtained by this treatment, however, is deficient in that it is not durable because the coating film has an extremely low adhering force and it is easily peeled off. To increase the adhering force of the film, it is desirable to heat the lens to a temperature of about 300.degree. C. However, the lens has poor heat-resistance and is not suitable for such heating. This is similar to the case in which an anti-reflection coating film of MgF2 or the like is to be applied to the lens. As a consequence an excellent lens product cannot be obtained.